The invention further relates to a method for producing a control surface element, in particular a spoiler, wherein an upper outer skin element is connected to a lower outer skin element, wherein at least one reinforcement rib is formed, wherein a core element made of a foam material is provided.
In the prior art various designs of air-flow aerodynamic control surface elements (designated in English as “control surfaces”) have been proposed by means of which flight control is accomplished. In the case of spoilers, usually a honeycomb core structure is provided which consists of supporting covering skins and a support core in honeycomb shape. However, this design has various disadvantages. It has been shown that the known control surfaces having a honeycomb structure are on the one hand liable to water ingress. On the other hand, a uniform impact protection cannot always be ensured. In the event of a collision between the walls of the honeycomb core, damage can occur. Furthermore, the air travel industry is continuously striving to achieve weight savings.
U.S. Pat. No. 6,234,423 B1 describes an elevator in which an upper outer skin and a lower outer skin are fabricated in two separate process steps. Each outer skin is formed by prepregs provided with core materials (i.e. fibre semi-finished products pre-impregnated with reaction resins) which is cured in an autoclave under pressure and heat. The upper outer skin is then adhesively bonded to the lower outer skin. During the manufacture of the outer skins, stiffening ribs are formed which have a self-closed cross-section which is filled with a form core. However, cavities are formed between the stiffening ribs. The known method is disadvantageously restricted to the fabrication of elevators, wherein the process sequence is additionally very complicated. The fabrication of the outer skins by prepregs in separate process steps is complex and expensive. Furthermore, the adhesive connections between the upper and lower outer skin are weak points which are susceptible to damage, in particular by impact loads. Accordingly, no effective force transfer from the upper side to the lower side of the control surface can be ensured. A further disadvantage consists in that the known elevator provides cavities between the reinforcement ribs which are susceptible to water ingress. As a result of these limitations, the known method does not meet the valid licensing requirements for air traffic.
US 2009/072090 A1 relates to a control surface element for an aircraft wing. In one embodiment, a reinforcement structure is disposed between outer skin elements. The reinforcement structure is located between two honeycomb cores. In addition, a free space is formed between two legs of the reinforcement structure
U.S. Pat. No. 3,775,238 A discloses an aileron with outer skin panels between which reinforcement ribs are disposed. The outer skin panels are constructed as composite components with a form core. In this prior art, free spaces also remain between the reinforcement ribs of the aileron.
DE 10 2008 013 759 A1 describes the manufacture of a fibre composite component in the form of an aerodynamic control surface which comprises stiffening elements enclosed by an outer skin. Here removable cores are introduced in a core mould in order to image the inner surface geometry of the fibre composite component. A fibre semi-finished product is then placed on the cores. The fibre composite component is finally fabricated in the RTM method. The cores are then removed. Accordingly in this prior art the cores are finally removed. For this purpose the core material of the cores is a fusible material whose melting point lies above the curing temperature of the matrix material or a curable substance which can be dissolved again subsequently by a suitable solvent and can be rinsed out from the subsequent component. Bores introduced subsequently in the outer skin or openings arranged in corner regions of the transverse ribs are used to remove the cores, and subsequently serve as drainage openings for the condensation water.
The document EP 1 227 035 A2 discloses a spoiler in which a fitting made of plastic material is provided. The spoiler has a lightweight core in the form of a honeycomb core which is covered with an upper cover layer and a lower cover layer. The lightweight core has a cut-out corresponding to the fitting.
In view of this, it is the object of the invention to eliminate or ameliorate individual ones or all the disadvantages of the prior art. The invention therefore in particular has the aim of creating a control surface element of the type specified initially which can be produced by a simplified method, in particular by an infusion method and facilitates the formation of the at least one reinforcement rib.
According to the invention, the reinforcement rib is disposed between two core segments of the core element.
Advantageously the reinforcement rib (or a semi-finished product to form the reinforcement rib) can be reliably fixed laterally by the core segments, i.e. in the longitudinal direction of the control surface element (or in the span width direction of the aircraft). Preferably a plurality of reinforcement or stiffening ribs spaced apart in the longitudinal direction of the control surface element are provided. The design according to the invention enables the fabrication of the control surface element in an infusion process in which a plastic in the liquid state is guided, in particular sucked, through a fibre semi-finished product in order to jointly produce the upper outer skin, the lower outer skin and the reinforcement rib. During fabrication the reinforcement rib can be precisely positioned and moulded by the core segments in contact therewith. Furthermore, the arrangement of the core element has the advantage that the control surface element is better protected from water ingress during operation. Furthermore, it is advantageous that the impact protection is improved. If the core element, apart from the at least one reinforcement rib between the core segments, extends over substantially the entire length (or extension in the longitudinal direction of the control surface element) of the intermediate space between the upper and the lower outer skin of the control surface element, water ingress along the control surface element can be reliably prevented. Furthermore, it is preferably provided that the core segments of the one- or multipart core element extend substantially over the entire width (i.e. its extension in the principal plane of the control surface element, substantially perpendicular to its longitudinal direction) of the intermediate space between the upper and lower outer skin. This has on the one hand the advantage that the reinforcement rib is fixed over its length. On the other hand, the protection against water ingress can be further improved. For the purpose of this disclosure the directional information “above”, “below”, “front”, “rear” relate to the installed state of the control surface element by reference to the preferred application in a spoiler. For other types of control surface elements, the directional information should be applied as appropriate.
In order to stabilize or fix the reinforcement rib (or a semi-finished product to form the reinforcement rib) during fabrication in the longitudinal direction of the control surface element, it is advantageous if the reinforcement rib comprises at least one web extending substantially perpendicular to the longitudinal direction of the upper outer skin element, wherein the one core segment is in contact with the one longitudinal side of the web of the reinforcement rib and the other core segment is in contact with the other longitudinal side of the web of the reinforcement rib. Preferably the web of the reinforcement rib is disposed substantially perpendicular to the upper outer skin element. In this way, the reinforcement rib is held reliably between the two core segments during fabrication, for example, in an infusion process.
The reinforcement rib can have different cross-sectional geometries. It is preferred however that the reinforcement rib has an open cross-section. In contrast to a closed cross-section, this means that the reinforcement rib does not have an interior closed on all sides. The reinforcement rib is therefore held in the longitudinal direction of the control surface element (i.e. in the span width direction of the aircraft wing) by the core segments which are in contact with the reinforcement rib.
In order to bring about force transmission between the upper side of the control surface element and the lower side of the control surface element, the web of the reinforcement rib preferably extends from the inner side of the upper outer skin element to the inner side of the lower outer skin element.
Preferably the core element substantially completely fills intermediate spaces formed between the upper outer skin element, the lower outer skin element and the at least one reinforcement rib. In this embodiment the control surface element is substantially free from inner cavities whereby any ingress of water can be reliably prevented. Incidentally the protection against impact loads in the installed state is also substantially improved. Compared to this, control surface elements in the form of spoilers having a honeycomb core on the one hand had the disadvantage that water can penetrate into the empty intermediate spaces. On the other hand, impact loads between the walls of the honeycomb structure could cause damage. Furthermore, the present embodiment has the advantage that the reinforcement rib (or the semi-finished product provided for this) is fixed on all sides during the manufacturing process.
According to a particularly preferred embodiment, a fibre composite element, in particular made of carbon-fibre-reinforced plastic is provided as reinforcement rib, which fibre composite element is disposed between two separate core segments of the core element. The fibre composite element is formed by a fibre semi-finished product, in particular a fibre scrim, a fibre woven fabric, a fibre mesh, a fibre knitted fabric, a fibre mat which is impregnated or infiltrated with a cured plastic, for example, epoxy resin.
In this embodiment it is favourable if the reinforcement rib comprises at least one upper flange extending substantially parallel to the principal plane of the upper outer skin element and arranged on the inner side of the upper outer skin element and/or a lower flange extending substantially parallel to the principal plane of the lower outer skin element and arranged on the inner side of the lower outer skin element, wherein the upper and/or the lower flange is disposed at an angle, preferably substantially at right angles to the web of the reinforcement rib. Preferably the reinforcement rib has a C-profile which is formed by the web with the flanges angled therefrom. This design enables a favourable removal (transfer) of load from the upper outer skin element with the upper-side outer surface and from the lower outer skin element with the lower-side outer surface into the reinforcement rib of the control surface element.
In order to arrange the upper or lower flange of the reinforcement rib substantially flush with adjoining sections of the upper or lower side of the core element, it is preferably provided that at least one of the core segments has a lower recess for the lower flange of the reinforcement rib on a lower side facing the inner side of the lower outer skin element and/or an upper recess for the upper flange of the reinforcement rib on an upper side facing the inner side of the upper outer skin element. In this way, the core element can substantially completely fill the intermediate space between the upper and lower outer skin element.
According to an alternative preferred embodiment, the core element is provided with seams made of fibre composite material to form the at least one reinforcement rib. Accordingly, in this embodiment the reinforcement rib is formed by a replacement structure of reinforcement seams. In this embodiment the reinforcement rib is also disposed between two core segments which are formed on both sides of the seam made of fibre composite material. As a result of the precisely fitting arrangement of the seams made of fibre composite material in through holes of the core element, the fibre semi-finished product is fixed for the formation of the reinforcement rib during fabrication. The reinforcement of foam with seams made of fibre composite material is known per se in the prior art. For example, a conventional method by means of which reinforced foam materials can be fabricated is known in the aircraft industry. As in this prior art, during the manufacture of the present control surface element a through-hole can be initially created in the foam material of the core element. Then a fibre bundle is disposed on the one side of the core element, then a hook-shaped needle is guided from the other side of the core element through the through hole and finally the fibre bundle is drawn through the through hole in the foam material. During manufacture the fibre bundle is impregnated with plastic in the liquid state in order to obtain the seams made of fibre composite material. In this embodiment the reinforcement rib is therefore integrated into the core element which is preferably formed in one piece. The fibre bundle preferably consists of a plurality of individual fibres or monofilaments.
In order to form the reinforcement rib in the core element, it is favourable if the seams of fibre composite material each extend from the upper side of the core element facing the upper outer skin element to the lower side of the core element facing the lower outer skin element. This embodiment additionally enables the arrangement of the seams in through-holes of the core element which extend from its upper side to the lower side.
In order to form the reinforcement rib in the transverse direction of the control surface element, it is favourable if a plurality of seams made of fibre composite material are disposed on a plane of the core element, the plane running substantially perpendicular to the upper or lower side. In this embodiment, the reinforcement rib is obtained by providing the core element along a vertical plane with a plurality of individual seams which in their entirety form the reinforcement rib in the transverse direction of the control surface element. Preferably individual seams are provided substantially over the entire width of the core element, i.e. over its entire extension perpendicular to the longitudinal direction of the control surface element. In order to form a reinforcement rib having a certain width (or extension in the longitudinal direction of the control surface element), it is favourable if a plurality of planes of the core element extending substantially perpendicular to the upper or lower side and arranged at a distance from one another are penetrated by seams made of fibre composite material. The width of the reinforcement rib is determined by the distance between the outer vertical planes with the seams when viewed in the longitudinal direction of the control surface element. The length of the reinforcement rib preferably substantially corresponds to the width of the core element which in the case of a spoiler is the extension in the depth or flight direction.
In this embodiment, it is furthermore favourable if a flat fibre composite element is disposed between the upper side of the core element and the inner side of the upper outer skin element in the region of the seams made of fibre composite material forming the reinforcement rib, which fibre composite element is preferably disposed substantially perpendicular to the reinforcement rib. Accordingly, another flat fibre composite element can be disposed between the lower side of the core element and the inner side of the lower outer skin element in the region of the seams made of fibre composite material forming the reinforcement rib, which other or further fibre composite element is preferably also disposed substantially perpendicular to the reinforcement rib. The flat fibre composite elements can be sewn to the core element. In this embodiment, a reinforcement structure having an I-shaped or C-shaped cross-section can be created, wherein the reinforcement rib forms the web, the flat fibre composite element forms the upper flange and the further fibre composite element forms a lower flange of the C- or I-shaped reinforcement structure.
For stability reasons it is favourable if the seams made of fibre composite material are arranged at an angle differing from 90° of preferably between 30 and 60°, in particular of substantially 45°, with respect to the upper or lower side of the core element.
In known control surface elements, in particular spoilers, a reinforcement or end strip of fibre composite material was disposed in many cases at the rear edge when viewed in the direction of flight (also designated as “trailing edge” in aircraft construction).
In the previously described embodiment, instead of the reinforcement strip, on a rear edge the core element can be provided with a reinforcing structure running substantially in the longitudinal direction of the upper outer skin element, which reinforcing structure is formed by further seams of fibre composite material. In this embodiment the core element is penetrated in the region of the rear edge or “trailing edge” by seams which together form the reinforcement structure in the longitudinal direction (i.e. in the case of a spoiler in the span width direction) of the control surface element. Preferably the reinforcement structure extends substantially over the entire length of the core element, i.e. substantially over its entire extension in the longitudinal direction of the control surface element. The further seams for the reinforcement structure at the rear edge of the core element can be configured like the seams for the reinforcement rib so that reference is made to the preceding explanations. Furthermore, here also seams can be provided at an angle of substantially 90° to the upper or lower side of the core element.
According to an alternative preferred embodiment, in order to form the reinforcement rib a form element separate from the core element is provided, which is provided with seams made of fibre composite material. In this embodiment, a form element of higher density or strength is preferably disposed between two core segments of the core element having lower density or strength compared to this. In order to obtain the reinforcement rib, seams of fibre composite material pass through corresponding through-holes of the form element.
The previously described embodiment in particular has the advantage that the form element of the reinforcement rib can consist of a form which is different from the form material of the core element or of the same form material with a higher density compared to this. Advantageously the form of the form element can be optimized in this way with regard to the introduction of seams made of fibre composite material or lower resin acceptance. Compared with this, the form material of the core element can be adapted to its intended usage. In order to achieve an integral control surface element, it is favourable if the upper outer skin element, the lower outer skin element and the at least one reinforcement rib consist of fibre composite material, in particular carbon-fibre-reinforced plastic, wherein the upper outer skin element, the at least one reinforcement rib and the lower outer skin element are interconnected via the plastic of the fibre composite material. Advantageously an integral connection is made between the upper outer skin element, the lower outer skin element and the interposed reinforcement rib which is substantially exclusively formed by the plastic of the fibre composite material. This means that the plastic in the liquid state impregnates a fibre semi-finished product by means of which the upper or lower outer skin element and the reinforcement rib is formed. As a result of the impregnation of the fibre semi-finished product with the plastic in the liquid state, a seamless bond between the upper or lower outer skin element and the at least one reinforcement rib is achieved. This embodiment brings about a substantial simplification compared with the prior art in which the upper and the lower outer skin element together with reinforcement ribs are fabricated separately and then adhesively bonded to one another (or bolted or riveted). Advantageously in the present embodiment such an adhesive, bolted or riveted connection between the upper and the lower outer skin element and the reinforcement rib can be dispensed with, whereby a structural weak point of the control surface element is eliminated. Furthermore, the drill holes possibly present in the prior art which form potential leaks for an ingress of liquid can be eliminated. In addition, the process sequence can be configured substantially more simply and rapidly.
In order to keep the weight of the control surface element as low as possible but ensure sufficient stability for the reinforcement rib, it is advantageous if the form material of the core element is formed from polymethacrylimide. Such a form is marketed for example under the trade name “Rohacell”. This form has a weight of 31 kilograms per cubic meter. However other hard forms can also be used for the core element which consist for example of polyvinylchloride or polyurethane.
In the method according to the invention, the reinforcement rib is positioned or formed between two core segments of the core element. This method has the advantages described previously for the control surface element to which reference can thus be made. It is essential for the invention that the reinforcement rib is fixed laterally during the manufacturing method between the core segments of the core element, thereby ensuring the exact positioning and formation of the reinforcement rib.
In order to produce the control surface element, preferably a fibre semi-finished product is disposed in the dry state on the core element in order to form the upper outer skin element, the lower outer skin element and the at least one reinforcement rib and is then impregnated with a plastic in the liquid state. In this embodiment, the upper outer skin element, the lower outer skin element and the at least one reinforcement rib are therefore produced in the same process step (“in one shot”). This means that the upper and lower outer skin element together with the reinforcement rib are joined together in the non-cured state of the plastic. It is therefore not necessary, as is usual in many cases in the prior art, to produce the upper outer skin element and the lower outer skin element in separate process steps by curing prepregs in an autoclave and then adhesively bonding, bolting or riveting them together in the cured state. It is not necessary to use an autoclave in the present embodiment. The curing of the impregnated fibre semi-finished product can take place in an oven with temperature supply or in vacuum.
In order to form the upper outer skin element, the lower outer skin element and the at least one reinforcement rib in one process step, it is advantageous if the fibre semi-finished product is arranged by means of a film in an infusion space which is connected to at least one supply line for the plastic in the liquid state and to at least one vacuum line, wherein by applying a negative pressure to the vacuum line the plastic in the liquid state is sucked in, wherein the fibre semi-finished product is impregnated to form the upper outer skin element, the lower outer skin element and the at least one reinforcement rib. Preferably the control surface element is therefore produced in an infusion process wherein the core element ensures that the fibre semi-finished product is held in the desired position for the at least one reinforcement rib when the plastic in the liquid state is guided through the fibre semi-finished product. Such infusion processes are known per se in the prior art.
Preferably a process known as MARI (“Membrane Assisted Resin Infusion”) is used which is described in the European Patent EP 2 681 037. When using this process, the fibre semi-finished product is disposed on a carrier mould wherein an infusion space sealed with respect to the carrier mould is formed with an airtight film. The infusion space is connected on the one hand to at least one supply line for the plastic in the liquid state and on the other hand to at least one vacuum line. A negative pressure is applied to the vacuum line in order to impregnate the fibre semi-finished product with plastic. The vacuum line has a membrane filter which is permeable to air and impermeable to resin in the dry state, which goes over into a substantially airtight state during an impregnation with resin. After complete impregnation of the fibre semi-finished product, the plastic enters into the vacuum line which has the membrane filter. When completely wetted with resin, the membrane filter closes whereby the air extraction is interrupted. In a preferred embodiment of the membrane filter which is known from sports clothing, a thin silicone skin is provided which is overstretched in the production process in such a manner that fine pores are produced which are permeable to air by impermeable to liquid, in the present case resin. In contact with resin, the pores gradually become closed so that the membrane goes over into the airtight state. The silicone skin can be connected to a carrier layer in the form of a woven fabric.
Furthermore it is also feasible to produce the control surface element using the injection process described in EP 1 181 149 B1.